1. Field of the Invention
This invention relates to an optical system used in observing a virtual image of an image displayed on an image display device.
2. Description of the Related Art
There has so far been proposed an optical system used in observing a virtual image of an image displayed on an image display device, such as an LCD (liquid crystal display). As an optical element capable of constructing this optical system to a small size, there has been proposed a Lippman volumetric hologram.
That is, if a light beam radiated from an image display apparatus is reflected by the Lippman volumetric hologram and the reflected light is observed, the Lippman volumetric hologram operates as a reflection type holographic optical element having a reflective power, such as a hologram lens, so that a viewer is able to observe a virtual image of a displayed image by the image displaying apparatus.
Meanwhile, in an optical system for observing a virtual image, in which the Lippman volumetric hologram is used as a reflection type holographic optical element as described above, there is raised a problem in connection with the aberration produced in the Lippman volumetric hologram. This amount of the aberration depends on the shape of the hologram, that is on the shape of a hologram substrate.
If the hologram is planar, as conventionally, there are occasions wherein the aberration generated on passage of the light beam through the holographic optical element is increased to render it impossible to realize sufficient performance as the optical system. The generation of the aberration becomes particularly outstanding when the F-number of the hologram lens is small.